1. Field of the Invention
The present invention is directed to a shock wave source of the type having an electromagnetic coil which is supplied with a high voltage pulse so as to rapidly repel an electrically conductive membrane disposed adjacent to the coil for generating a pressure pulse.
2. Description of the Prior Art
A shock wave source, or shock wave tube, of this type is described, for example, in German OS No. 35 02 751. The use of such a shock wave source in medical technology for disintegrating calculi disposed in the body of a patient is described, for example, in German OS No. 33 12 014. As a result of the high pressure pulse of, for example, 100 bar generated by such a shock wave source, the components of the shock wave source are highly stressed given repeated discharges and shock wave emissions. In particular, the membrane is exposed to strong electromagnetic and mechanical forces, which can lead to a premature material fatigue. In conventional shock wave sources, this membrane is compact and consists of material having high electrical conductivity and high mechanical strength. The membrane is clamped firmly at its edge around the entire perimeter thereof, as described in German OS No. 35 02 751. The membrane is preferably homogeneous, and may consist of a metal such as, for example, copper or an alloy having high conductivity such as bronze or silver-bronze. The membrane may alternatively consist of a carrier, for example, beryllium bronze, or a polymer having an applied coating such as, for example, a galvanic layer of silver or copper.
The duration of the initial acoustic pressure pulse generated by shock wave sources of this type is significant for various reasons. In theory, a shortening of the duration of this initial pressure pulse would result in a shortening of the approach path leading to the formation of a shock wave, a smaller focus zone (-6 dB zone), a higher focusing factor, i.e., a higher peak pressure at the focus given a prescribed initial pressure, lower electrical and thermal stresses on the shock wave source for achieving a defined peak pressure, a relatively slight quantity of acoustic energy output into the body of the patient, and a change in the predominantly effective mechanism of calculus destruction toward an "erosion" of the calculus instead of a "disintegration" as occurs in the case of a relatively long initial pressure pulse. Thus shortening the duration of the initial acoustic pulse provides a number of significant advantages in comparison with conventional techniques.
The duration of the initial pressure pulse is determined primarily by the duration of the discharge current of a capacitor which is connected to the coil, and thus by the electrical properties of the discharge circuit. Another factor contributing to the duration of the initial pressure pulse is the mass of the electrically conductive membrane. If, using a metallic membrane, the impedance of the discharge circuit is reduced by using a capacitor having a smaller capacitance (for example, 0.25 .mu.F instead of 1 .mu.F), the duration of the discharge current will be noticeably shorter but the membrane can no longer completely execute the necessary movement based on the current curve due to its inertia. A pressure pulse having a longer duration and a lower amplitude than in the ideal case results. Using a coated membrane, by contrast, the electrically conductive layer, having a thickness of about 30 through 50 .mu.m is not strong enough to permit the eddy currents induced by the coil to reach full strength, thus also resulting in a reduced efficiency. Practical manufacturing problems do not permit simply using a thicker galvanic layer having the same conductivity as the compact metal layer because joining techniques other than electroplating would be necessary, and such other joining techniques cannot accept the same degree of mechanical loading as an electroplated layer. In general, therefore, efforts to improve the electrical operation of the membrane cause mechanical disadvantages, and vice versa.